This application is a continuation-in-part of application Ser. No. 10/064,121, filed Jun. 12, 2002, now U.S. Pat. No. 6,761,837.
This invention was first conceived or reduced to practice in the performance of work under contract DE-FC26-99FT40632 awarded by the United States Department of Energy. The United States of America may have certain rights to this invention.
The present invention relates to phosphors comprising oxides of rare-earth and Group-IIIB metals activated with trivalent europium and having improved quantum efficiency. In particular, the present invention relates to such phosphors having improved absorption in the ultraviolet wavelengths. The present invention also relates to fluorescent lamps containing such phosphors.
A phosphor is a luminescent material that absorbs radiation energy in a portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. Phosphors of one important class are crystalline inorganic compounds of very high chemical purity and of controlled composition to which small quantities of other elements (called “activators”) have been added to convert them into efficient fluorescent materials. With the right combination of activators and host inorganic compounds, the color of the emission can be controlled. Most useful and well-known phosphors emit radiation in the visible portion of the electromagnetic spectrum in response to excitation by electromagnetic radiation outside the visible range. Well-known phosphors have been used in mercury vapor discharge lamps to convert ultraviolet (“UV”) radiation emitted by the excited mercury vapor to visible light. Other phosphors are capable of emitting visible light upon being excited by electrons (used in cathode ray tubes) or X rays (for example, scintillators in X-ray detection systems).
The efficiency of a lighting device that uses a phosphor increases as the difference between the wavelength of the exciting radiation and that of the emitted radiation narrows. In low-pressure mercury discharge lamps (also commonly known as fluorescent lamps), excited mercury atoms in the discharge, upon returning to the ground state, mainly emit UV radiation having wavelength of 254 nm (about 12% of the emitted radiation having wavelength of 185 nm). Ideal phosphor for mercury discharge lamps should absorb the 254 nm and 185 nm UV radiation strongly and convert the absorbed radiation efficiently. Effort, therefore, has been expended to produce phosphors for these lamps to be excited by radiation having wavelengths as close to 254 nm as possible. A plurality of phosphors is typically included in a low-pressure mercury discharge lamp to provide white light that simulates sun light. Different blends of phosphors can produce fluorescent lamps with different color temperatures. The color temperature of a light source refers to the temperature of a blackbody source having the closest color match to the light source in question. The color match is typically represented and compared on a conventional CIE (Commission International de I' Eclairage) chromaticity diagram. See, for example, “Encyclopedia of Physical Science and Technology,” Vol. 7, 230–231 (Robert A. Meyers (Ed.), 1987). Generally, as the color temperature increases, the light becomes bluer. As the color temperature decreases, the light appears redder. Typical incandescent lamps have color temperature of about 2700 K while fluorescent lamps have color temperature in the range of 3000–6500 K. When the point representing the light source is not exactly on the black body locus of the CIE chromaticity diagram, the light source has a correlated color temperature, which is the temperature on the black body locus which would give nearly the same color to the average human eye.
In addition to correlated color temperature, color rendering index (“CRI”) is another important characteristic of the light source. CRI is a measure of the degree of distortion in the apparent colors of a set of standard pigments when measured with the light source in question as opposed to a standard light source. CRI depends on the spectral energy distribution of the emitted light and can be determined by calculating the color shift; e.g., quantified as tristimulus values, produced by the light source in question as opposed to the standard light source. Under illumination with a lamp with low CRI, an object does not appear natural to the human eye. Thus, the better light sources have CRI close to 100. Typically, for color temperatures below 5000 K, the standard light source used is a blackbody of the appropriate temperature. For color temperatures greater than 5000 K, sunlight is typically used as the standard light source. Light sources having a relatively continuous output spectrum, such as incandescent lamps; typically have a high CRI; e.g., equal to or near 100. Light sources having a multi-line output spectrum, such as high pressure discharge lamps, typically have a CRI ranging from about 50 to 80. Fluorescent lamps typically have a CRI in the range of 75–85. Typically, fluorescent lamps have higher color temperature, but lower CRI than incandescent lamps. In general lighting applications, it is desirable to provide light sources having color temperature in the range of 4000–6000 K; i.e., in the range of color temperature of fluorescent lamps.
Typically, a low-pressure mercury fluorescent lamp uses a blend of three phosphors absorbing in the UV range near 254 nm and emitting in the blue, green, and red regions. The colors of the emitted light are blended to provide white light. A commonly used red-emitting phosphor has been Y2O3:Eu3+, which has a quantum efficiency close to 100%. However, this phosphor requires very high purity Y2O3, which is expensive.
Therefore, there is a continued need to provide UV-absorbing, red-emitting phosphors that is less expensive to make. In addition, it is very desirable to provide UV-absorbing, red-emitting phosphors that also have high quantum efficiency. It is also very desirable to provide such phosphors in gas discharge lamps.